From what people have deciphered from the trailers it appears that the rain makes people older/moves time forward which is why you see the one guy get super old when the rain touches him in the trailer from last year. Norman Reedus character seems to be immune from this. Not sure how the babies play into it. Maybe the babies are clones and you can transfer your consciousness into them and then when the rain hits the baby you rapidly age and become an adult.

“Early-build players have found themselves inexorably drawn in as Death Stranding introduces the plot, establishes characters, reveals narrative tension, and resolves the story over the course of 120 minutes, and it’s only a matter of time before the greater public latches onto this sort of storytelling experience—one that’s bold enough to challenge convention by taking autonomy away from gamers.”